Field of the Invention
The present invention relates to an optical amplifier that reduces signal loss by reducing crosstalk.
With the rapid development of multimedia networks, demand for information is increasing remarkably and therefore there is a need for further improvement in capacity and network flexibility. These needs may be addressed by improving the main optical transmission system for concentrating information capacity.
The wavelength division multiplexing (WDM) system is currently the most effective system for meeting the above-described demand and it is now intended for commercial use mainly in North America. In the WDM transmission system, an optical fiber amplifier is an essential device.
The present invention is applied to a structure of an optical amplifier to amplify a plurality of multiplexed bands. More specifically, the present invention allows the band of an optical amplifier to be widened.
2. Description of the Related Art As a method of widening the bandwidth of a repeater utilizing an optical fiber amplifier, there is provided a structure in which a plurality of bands are respectively amplified with exclusive optical amplifiers. An optical multiplexer and an optical demultiplexer are respectively provided at the input and output ends for parallel amplification.
Referring to FIG. 1, which is a block diagram of a multiple-band-transmission system with a parallel amplifying structure, including a transmitting part 1-1 and a receiving part 1-2, a demultiplex coupler 100 demultiplexes the C-band (1.53 to 1.56 xcexcm) and the L-band (1.57 to 1.60 xcexcm). A multiplex coupler 200 multiplexes the C-band and L-band. An optical amplifier 700 amplifies the C-band and an optical amplifier 800 amplifies the L-band.
The optical amplifying repeater of FIG. 1 has a multiple bandwidth parallel amplifying structure such that crosstalk exists in the demultiplex coupler 100 and the multiplex coupler 200. Namely, in demultiplex coupler 100, the C-band output is in the range of 1.53 to 1.56 xcexcm. Ideally, this output is at the port on the C-band side of optical amplifier 700, but when isolation for rejecting the L-band is set to a higher value, the insertion loss of demultiplex coupler 100 tends to become large and therefore loss of the signal transmitted (main signal light) becomes large, resulting in deterioration of an SN (signal to noise) ratio of the communication system as a whole.
Conversely, when isolation of the demultiplex coupler 100 is set to a relatively low value in order to maintain lower loss for the main signal, a problem arises in that the light of the L-band (1.57 to 1.60 xcexcm) is output as the crosstalk light (leakage light), also at the port of the optical amplifier 700 side of the demultiplex coupler 100 which naturally has to output only 1.53 to 1.56 xcexcm of the C-band. Similarly, a certain amount of C-band leakage light is also incident to the port on the optical amplifier 800 side on the L-band side of demultiplex coupler 100.
If crosstalk light occurs, it will result in detection level error in an optical input monitor located within optical amplifier 700 on the C-band side and optical amplifier 800 on the L-band side. In particular, when the optical amplifiers 700,800 are controlled based on such a detection level (gain constant control or AGC control) and input off monitor (shut-down detection control) control error occurs because an L-band optical light leaks toward the C-band side and a C-band light leaks toward the L-band side.
For example, we will discuss the case of AGC control as applied to optical amplifier 700, noting that the AGC control is performed in a similar manner at optical amplifier 800. The optical power level of the exciting light source of optical amplifier 700 is controlled so that the gain becomes constant by detecting an input to output ratio. The input light monitor of optical amplifier 700 detects an input level in which a crosstalk light is added to the optical power of the wavelength band of the main element and the monitor of an output light of optical amplifier 700 can neglect the crosstalk light because optical amplifier 700 does not amplify the light outside the amplifying bandwidth.
Therefore, since the influence of the crosstalk light on the input monitor and output monitor is different, correct detection of gain, namely, AGC control, is not conducted for a light within the amplifying wavelength band of the optical amplifier 700.
Moreover, when a shut-down detection is to be performed to prevent surge at the time of recovery by monitoring the input light and stopping pumping of optical amplifier 700 or fixing pumping to a particular value if there is no optical input, it may occur that the shut-down condition of the light cannot be detected due to the influence of the crosstalk light despite the fact that the light in the band of the optical amplifier 700 is shut-down. Moreover, such crosstalk may also occur in multiplex coupler 200.
In the case when the output of optical amplifier 700 and the connectors of the transmission lines are disconnected, to assure operator safety, it is required that such a condition be automatically detected in order to stop or decrease the output of optical amplifiers 700,800.
In general, the output of optical amplifier 700 is stopped or decreased by obtaining an amount of reflection with detection of the levels of the reflected light and output of light at the output of each optical amplifier 700,800 in order to determine a difference in reflection amount when the connectors are connected or when they are disconnected.
When the connectors are disconnected, the light, which is subjected to Fresnel reflection (xe2x88x9214 dB) at the connector, and loss equal to two times the loss of the multiplex coupler 200, is returned to optical amplifiers 700,800. Assuming the maximum multiplex filter loss, the connector opening detection threshold value is set to a value less than the amount of reflection.
Meanwhile, when the connectors are connected, considering two times the loss of multiplex coupler 200 as the amount of reflection (determined by reflection attenuation of the connector and Rayleigh scattering in the transmission line), the maximum amount of reflection is assumed and the threshold value of the connector connecting condition is set higher than the maximum reflection amount.
When isolation of optical multiplex coupler 200 is set to a relatively low value, the light of the other bands return to optical amplifiers 700,800 as the crosstalk light (leakage light). Therefore, a problem arises in that the amount of reflection when the connectors are connected increases, a difference of setting values of threshold values when the connectors are opened and connected becomes relatively small, and it becomes difficult to set the threshold value and thereby normal operation is lost.
In the present invention, in order to overcome the crosstalk of the filter for dividing the band and for amplifying the light in the multiple wavelength bandwidths during optical communication, the optical monitor for each band of the optical amplifier is given the characteristic of rejecting the crosstalk light.
Thereby, rejection of the crosstalk light can be realized without increasing the loss of the main signal and accordingly the structure of the optical amplifier/repeater, which maintains the controllability of the optical amplifier without determining the signal characteristics, is realized.
Additional objects and advantages of the invention will be set forth in part in the description which follows